Paper based lithographic printing plates and methods for making the same have been well known for a considerable period of time. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic printing plate is first imaged in a known manner with typed, written or drawn copy material to be reproduced. The image may also be obtained in other ways, for instance by xerography; e.g. Electrofax (Trademark, Radio Corp. of America) and Xerox (trademark, Xerox Corp.). The grease-receptive imaging material employed makes the imaged areas ink receptive and water-repellent (i.e. hydrophobic). The remaining non-imaged surface is water-receptive and ink-repellent (i.e. hydrophilic).
The imaged plate is placed on a plate cylinder of an off-set duplicating press. The over-all surface of the plate is then treated with an aqueous wet-out liquid which wets all portions of the plate except those areas that have been imaged and are water-repellent. The press inking roll then passes over the surface of the plate and deposits a film of ink only upon the ink-receptive imaged areas. In the printing operation, the ink from the imaged areas is transferred in reverse to a rubber off-set blanket which in turn prints directly onto a paper sheet so as to form a copy.
A number of different coating formulations and techniques have been utilized in the past to provide paper based sheets with suitable lithographic printing surfaces. In this regard, the use of colloidal silicas is known. Prior U.S. Pat. No. 3,055,295 describes the preparation of lithographic printing plates comprising a base having adhered thereto an insolubilized coating comprising a pigment and the reaction product of a hydrophilic colloid and a cross-linking agent, the pigment containing a colloidal silica having a particle size between about 7 and about 30 millimicrons. Exemplary silicas listed are those marketed by E. I. duPont de Nemours and Co. under the trademarks Ludox S. M., Ludox L. S. and Ludox H. S.; and also by the Monsanto Chemical under the trademark Syton 200. Exemplary hydrophilic colloids are polyvinyl alcohol, unmethylolated polyacrylamide and others. The patent also lists a number of cross-linking agents which can be employed. Although the purpose of the cross-linking agent is to insolubilize the coating, it is stated in the patent that the colloidal silica greatly assists the insolubilization of the hydrophilic colloid so that a substantially lower temperature can be used to insolubilize the colloid. Other representative patents employing colloidal silicas are U.S. Pat. Nos. 2,681,617; 2,741,981; 3,220,346; 3,254,597; 3,270,667; and 3,455,241.
In the above patents, specific colloidal silicas, where given, are the same as those set forth in U.S. Pat. No. 3,055,295. All of the silicas are characterizable as being negatively charged and to applicant's knowledge a positively charged silica has not heretofore been employed in the preparation of a lithographic printing plate. One reason for this is believed to be that conventional pigments and insolubilizers employed in the preparation of lithographic face coatings are strongly alkaline, and thus incompatible with a positively charged silica.
The coatings of the aforementioned prior patents suffer from a number of disadvantages. For one, it has traditionally been necessary to apply to the printing plate base, particularly where the base is paper or of a cellulosic material, one or more barrier coats offering water resistance and compensating for surface unevenness. Obviously, the application of such coats adds to the cost of the printing plate.
Another disadvantage associated with conventional colloidal silica containing face coats is the short life of such coats in the absence of special barrier coats. Also the use of conventional negatively charged silicas requires employing in the face coat formulation additional additives such as pigments, insolubilizers, cross-linking agents, and other materials to achieve the desired properties of clean copy, good toning, good imaging, stop-go properties and others commonly associated with high quality planographic printing plates. Also, it has conventionally been necessary to take special steps to prevent curling or wrinkling of the printing plates while on a press.